1. Field
Exemplary embodiments relate to a platinum and cobalt/copper-based multilayer thin film, a fabrication method thereof, and the use thereof in magnetic random access memory (MRAM).
2. Description of the Related Art
In an effort to overcome volatility of dynamic random access memory (DRAM), which is a semiconductor memory device that is widely used in electronic devices such as, personal computers (PCs) and mobile phones, studies on magnetic random access memory (MRAM) having non-volatile memory characteristics have been actively conducted. Non-volatile memory can retain data even when a power supply is interrupted. Particularly, in recent years, the integration density of dynamic random access memory reached a limit, and thus magnetic random access memory has been considered as a substitute for dynamic random access memory. Therefore, in related industrial fields, research and development of the MRAM has been actively conducted.
Studies on magnetic random access memory have been conducted since the early 2000s. The early studies were mainly focused on changing the resistance of tunneling magneto-resistance devices by reversing magnetization using a magnetic field created by application of an electric current. However, MRAM devices based on this tunneling magneto-resistance have a shortcoming in that, as the size of the devices decreases, the amount of writing current greatly increases, making it difficult to realize a large-scale, densely integrated memory.
Due to this shortcoming, an MRAM technology based on spin-transfer torque magnetization switching was introduced. It is a type of current-induced magnetization switching, and is based on a method of switching magnetization using a spin-transfer torque (hereinafter referred to as SU) generated by applying a current to a magnetic thin film. The MRAM based on this method is referred to as STT-MRAM. Spin-transfer torque magnetization switching provides various advantages, including high integration density, wide write window and low power consumption, compared to existing magnetic field-induced magnetization switching.
Prior studies on the STT-MRAM were focused mainly on magnetic tunnel junctions (hereinafter referred to as MTJs) with in-plane magnetic anisotropy. Recently, in-plane magnetic tunnel junctions (iMTJs), which have a relatively low critical current density while maintaining their thermal stability in nanosized magnetic cells, were also developed. Such results were mostly obtained in MgO-based structures having an exchange-coupled trilayer including a free layer and a pinned layer, but a MTJ that requires a lower critical current density (e.g., 1 MA/cm2 or less) is required to realize a highly integrated MRAM device for commercial use.
In view of this disadvantage of iMTJ, an MJG with perpendicular magnetic anisotropy (hereinafter referred to as PMA) has a very big advantage in that the critical current density required for magnetization switching is low. This is because the iMTJ requires additional torque to overcome a demagnetizing field (2n Ms, where Ms=saturation magnetization) during magnetization switching, and thus it is difficult to lower the critical current density. For this perpendicular MTJ (pMTJ), it is most important to develop materials and structures, which have excellent PMA properties (PMA energy density=about 107 erg/cc). However, from the view point of magnetostatic energy, PMA should overcome a very high demagnetizing field, and thus it is fundamentally difficult to develop materials and structures, which have excellent PMA properties.
High-density magnetic random access materials are required to have strong perpendicular magnetic anisotropy. This property should be achieved in very thin films having a thickness of 3 nm or less. Perpendicular magnetic anisotropy is largely divided into one caused by interfaces and one caused by bulk properties. Until now, four kinds of materials with perpendicular magnetic anisotropy have been mainly studied, including rare earth-3d transition metal amorphous alloys [N. Nishimura et al., J. Appl. Phys. 91, 5246 (2002).], intermetallic compounds, such as FePt and CoPt, which have the L10 structure [T. Shima et al., Appl. Phys. Lett. 80, 288 (2002)], multilayer thin films such as [W. B. Zeper et al., J. Appl. Phys. 70, 2264 (1991)], and CoFeB/MgO interfaces [S. Ikeda et al., Nature Mater. 9, 721 (2010)]. It is understood that the former two materials have intrinsic bulk properties, and the latter two materials have perpendicular magnetic anisotropy at the interfaces.
However, rare earth-3d transition metal amorphous alloys have issues in that the PMA energy density is insufficient and in that crystallization occurs even at a relatively low temperature (about 300° C.) to rapidly reduce the perpendicular magnetic anisotropy (PMA) properties. Also, multilayer thin-film structures such as CoPd and CoPt have sufficient PMA energy density, but have an issue in that the structure thereof is not maintained at a temperature ranging from about 350° C. to 500° C., which is used in magnetic random access memory fabrication processes. Thus, the PMA properties are reduced or lost. CoFeB/MgO interfaces have issues in that perpendicular magnetic anisotropy is exhibited only at a very thin CoFeB thickness, generally 1.5 nm or less, and in that the distribution of anisotropy is not good. However, intermetallic compounds such as, FePt and CoPt, which have the L10 structure, are currently known as materials having the best characteristics, since the PMA energy density is sufficiently high and the temperature characteristics are also good. However, the intermetallic compounds with the L10 structure also have an issue in that these compounds are not suitable for temperature conditions that are used in current memory device processes, since a temperature higher than 600° C. is required to form an intermetallic compound having a high long-range order which is known as the most important factor for perpendicular magnetic anisotropy. Additionally, there is a problem in that it is not easy to design a seed layer and a buffer layer, which are required to form the (001) texture essential for perpendicular magnetic anisotropy.
Accordingly, due to the above-described concerns, there is a need for a new material and structure which is suitable for use at the heat treatment temperature that is used in current memory fabrication processes, and at the same time, has sufficient perpendicular magnetic anisotropy density, and can be used for high-density magnetic random access memory. Embodiments relate to the fabrication of a multilayer thin film comprising platinum, cobalt and copper, which has perpendicular magnetic anisotropy and low saturation magnetization, and at the same time, is suitable for use at the magnetic random access memory process temperature (350° C. to 500° C., and suitable for serving as a multilayer thin film in magnetic random access memory.
Specifically, an embodiment is intended to reduce the influence of stray fields in high-density magnetic random access memory cells by greatly reducing saturation magnetization while maintaining perpendicular magnetic anisotropy by replacing a portion of cobalt in a platinum-cobalt multilayer thin film with copper. This magnetic multilayer thin film has properties suitable for use in a high-density magnetic random access cell, particularly a pinned structure in the cell.